(T1A06) Interference in 70 cm Band

This question is a new item from the recently released 2014-2018 VEC Technician Class question pool. It is covered in the new HamRadioSchool.com Technician License Course for 2014-2018. The new question pool goes into effect on July 1, 2014.

T1A06from the Technician License Course Section 2.4, Talking to the World:Which of the following services are protected from interference by amateur signals under all circumstances?

A. Citizens Radio ServiceB. Broadcast ServiceC. Land Mobile Radio ServiceD. Radionavigation Service

This question goes hand-in-hand with a couple of others from the 2014-2018 question pool: T1A14 asks what you must do if you find you are interfering with a radiolocation station outside of the United States, and T1B08 asks about the implications of US amateurs having secondary privileges on some portions of the 70 cm band. Let’s see how these questions are interrelated.

The United States and Canada utilize the 70 cm RF spectrum differently, particularly in the range of 420 – 430 MHz. In the US this range of the 70 cm band is designated for amateur television (ATV) transmissions (See band plan for 70 cm). However, along the northern border with Canada the US also employs some radar sites that use this range and that may be interfered with by amateur transmissions. Further, the US may utilize radionavigation with this frequency range, particularly with fixed wing aircraft. In Canada (and in some northern US cities) this portion of the 70 cm band is allocated to other radio services that operate land mobile and fixed stations that can also be interfered with by amateur signals.

This situation requires some special coordination between the US and Canada for the use of the lower end of the 70 cm band, and that is the topic of these question pool items. What, you ask, does the FCC say to amateurs in Part 97 about this messy situation?

Part 97.303(m) — In the 70 cm band:

(1) No amateur station shall transmit from north of Line A in the 420-430 MHz segment. See §97.3(a) for the definition of Line A. (See link item #30, or map below)

(2) Amateur stations transmitting in the 420-430 MHz segment must not cause harmful interference to, and must accept interference from, stations authorized by the FCC in the land mobile service within 80.5 km of Buffalo, Cleveland, and Detroit. See §2.106, footnote US230 for specific frequencies and coordinates.

(3) Amateur stations transmitting in the 420-430 MHz segment or the 440-450 MHz segment must not cause harmful interference to, and must accept interference from, stations authorized by other nations in the fixed and mobile except aeronautical mobile services.

The practical upshot of this for the US Technician Class licensee is:

1) You have secondary privileges on some portions of the 70 cm band, so you must avoid interfering with any non-amateur transmissions you may receive in the band.
2) You must cease any operations on 70 cm band (or other bands) that interfere with radionavigation services.
3) You may not transmit in the 420 – 430 MHz segment of the 70 cm band if you are in the US north of Line A.

Where’s Line A exactly, you ask? Here’s a coarse depiction of Line A on a map. If you are near this geographic area, check more closely to make sure you do not transmit between 420 – 430 MHz while north of this line. Notice also “Line C” in Alaska that has similar restrictions.

And, unless you are transmitting amateur TV, you probably won’t be using the 420 – 430 MHz portion of the band anyway. Thus, Line A is of lesser or no consequence for most common 70 cm band FM, SSB, or digital mode operators.

The correct response toT1A06, “Which of the following services are protected from interference by amateur signals under all circumstances?” is “D. Radionavigation Service.”